1. Field of the Invention
The present invention relates to a control unit for a printing apparatus having a plurality of heat sources, each heat source being operable at an individual power level. The control unit is adapted to control the power supplied to the heat sources such that, at each instant, the sum of the delivered individual power levels is less than or equal to a maximum allowable power. The present invention also relates to a printing apparatus comprising a control unit of this type.
2. Description of Background Art
Such a control unit may be applied in a printing system in which several sub-parts require heating. Heat sources, such as resistors are provided in the vicinity of the sub-parts to be heated and have to be supplied with electrical power (i.e. electrical current). Advantageously, many heat sources are connected to the same power supply unit. The control unit may be applied in an ink jet printing system, for example a hot melt ink jet printer.
A control unit of the type described above is known from EP 0987605. The heaters are divided in two groups, so that the power consumptions of the two groups are about the same. The power supply controller controls the power supplied to the heater of the first group throughout a first time. During the first time, when the temperature of the object heated by the heater of the first group falls below a preset lower limit temperature, power is supplied to the heater of the first group. When the temperature of the object heated by the heater of the first group reaches the preset upper limit temperature, the power supply is disconnected from the heater of the first group. These events may be repeated throughout the first time. After the first time has passed, the switch for the heater of the first group is turned off. The power supply controller then controls the power supplied to the heaters of the second group throughout a second time. After the second time has passed, the power supply to the heater of the first group is controlled again throughout the first time.
A drawback of the control unit according to EP 0987605 is that the temperature of the objects heated by the heaters of either group fluctuates considerably during time if they contain less thermal mass, since the controller is based on a hysteresis control principle. When the known controller is incorporated in an ink jet printing system, this may lead to inconsistent print results in time. Relatively large fluctuations of the temperature of the ink have a particularly negative impact on print results. Furthermore, the heaters have to be divided in fixed groups. Therefore, the proposed algorithm dynamically allocates power to the heaters.